Natural gamma-ray elemental concentrations provide information that is valuable for petrophysical purposes. For example, thorium and potassium are useful for mineral identification, especially when combined with other logging measurements. As a further example, zones with high uranium concentrations can be used to identify volcanic zones, and in many organic shale reservoirs, uranium content often correlates with total organic carbon content. Various methods have been developed to derive these elemental concentrations from pulse-height natural gamma-ray spectra. However, deviations from the desired calibration can occur for a variety of reasons. Voltage drifts caused by changes in electronics due to changes in temperature can lead to deviations in both gain and channel offset. Similarly, light output from most scintillation crystals varies with changes in crystal temperature which, in turn, affects the gain of the detector system. In addition, aging of the photomultiplier cathode can cause the spectrometer gain to change over time. For some detector systems a phenomenon known as photomultiplier fatigue occurs when large counting rate changes occur over short intervals of time. In well logging, spectrometers are often operated for long periods of time, and it is not uncommon for the gain to change because of some or all of these effects.
A variety of techniques have been developed in an attempt to compensate for departures from the desired energy scale calibration of gamma-ray spectroscopy measurements. Some conventional techniques involve continuously monitoring the location of a photopeak, requiring that one or more identifiable photopeaks be present in the observed spectra. Other techniques introduce a known source of gamma rays into the detector system to be superimposed on the spectrum produced by gamma rays external to the spectrometer. However, small inaccuracies in determining the known source's location in the spectrum can lead to gain adjustments that magnify errors at larger gamma ray energies. Further, adding chemical sources often creates a number of safety, security, regulatory, and environmental concerns.